1. Field of the Invention
The present invention relates to a mobile station and a transmission power control method in the mobile station.
2. Description of the Related Art
In recent years, according to rapid spread of the Internet, and to the space diversity and increasing capacity of information communication, and further according to a trend toward the development of the next generation Internet, the research and development of next generation radio access system for realizing high speed radio transmission in mobile communication have been energetically performed.
In such high speed radio communication, the development of technique for space diversity is a key for realizing more efficient communication in a same radio environment.
In particular, a transmission technique for space diversity, by which signals transmitted from a plurality of antennas on the base station side are received and synthesized by a reception terminal (mobile station), is mandatorily prescribed by the standard in the 3GPP (Third Generation Partnership Project).
In particular, a closed loop transmit diversity, by which a mobile station determines the phase of a signal transmitted from a base station and feedbacks the phase rotation information to the base station, has a high diversity gain, and hence has been widely used in the present third generation communication.
Further, in the next generation high speed communication transmission, the adaptive modulation and channel coding (AMC) in which the throughput is changed according to the reception environment of a reception terminal, and a hybrid ARQ (Automatic Repeat Request) which performs the packet retransmission and synthesis, are applied. Thus, a system configured to perform communication only with a cell in a best reception environment, is adopted, unlike the conventional W-CDMA technique which simultaneously effects connection with a number of cells.
FIG. 6 and FIG. 7 are conceptual diagrams for explaining phase states in the case of adopting the transmit diversity. FIG. 6 represents an open loop transmit diversity in which feedback information from a mobile station is not used, and FIG. 7 represents a cosed loop transmit diversity in which the phase of a transmission signal, and the like, is controlled on the basis of feedback information transmitted from a mobile station. The both modes are adopted in the W-CDMA system standardized in the 3GPP.
In FIG. 6, when a same signal TS is transmitted as signals TS1 and TS2 from two antennas (not shown) of a base station, the signals reach a mobile station (not shown) as a signal RS1 and a signal RS2 through a radio space which is a propagation path.
A diversity combining gain can be obtained by combining the signal RS1 and the signal RS2 in the mobile station. However, the diversity combining gain may not be obtained because the phase of the signal is rotated in the radio propagation path.
That is, in the case of FIG. 6, the signal TS1 reaches the antenna of the mobile station as the signal RS1 with no phase rotation, while the signal TS2 reaches the antenna of the mobile station as the signal RS2 with a phase rotated by +90 degrees.
Therefore, when the signal RS1 and the signal RS2 are synthesized, the diversity combining gain is deviated, so that a prescribed gain (ideally two times) cannot be obtained.
On the other hand, in the cosed loop transmit diversity shown in FIG. 7, the amount of phase rotation in the radio propagation path is culculated separately for each transmission antenna in the mobile station. Feedback information, which requires the each antenna to transmit a signal with a phase shifted beforehand on the basis of the culculated result, is transmitted from the mobile station to the base station.
That is, when two signals TS1 and TS2 are transmitted from the two antennas through the radio propagation path, the signal TS2 reaches the antenna of the mobile station as the signal RS2 with the phase rotated by +90 degrees, as described above with reference to FIG. 6. However, in the cosed loop transmit diversity, the signal TS2 is transmitted as a signal RS2 with a phase rotated beforehand by −90 degrees with respect to the signal TS1. Thereby, the phases of the signal RS1 and the signal RS2, which reach the mobile station, are made to coincide with each other, so that the prescribed gain can be obtained.
Note that in the 3GPP, the open loop transmit diversity includes the STTD (Space Time block coding based Transmit Diversity) and the TSTD (Time Switched Transmit Diversity), and in the cosed loop transmit diversity, there are specified the mode 1 which controls only the phase, and the mode 2 which controls both the phase and the amplitude.
As will be described below, the present invention relates to a problem caused in the case where the cosed loop transmit diversity is applied, but the present invention can be applied regardless of the difference between the above described mode 1 and mode 2. Thus, in the following description, the above described modes are not specifically distinguished.
FIG. 8 is a conceptual diagram showing a state of communication in which conventional individual channels are used. A plurality of base stations 210 and 220 (for convenience of explanation, two base stations are shown in FIG. 8) and a mobile station 100 are connected to each other by uplink control channels 11 and 12, downlink control channels 21 and 22, and downlink data channels 31 and 32.
The timing, rate, and the like, which relate to data transfer in the downlink data channel 31 are adjusted by control signals of the uplink control channel 11 and the downlink control channel 21. Similarly, the timing, rate, and the like, which relate to data transfer in the downlink data channel 32 are adjusted by control signals of the uplink control channel 12 and the downlink control channel 22.
FIG. 9 is a conceptual diagram showing a state of communication in which data transfer is performed by using a shared channel as in the HSDPA proposed by the 3GPP.
In the HSDPA (High Speed Downlink Packet Access), a high-speed physical downlink shared channel 31 (HS-PDSCH) is used as the downlink channel used for information (data) transfer from the base stations 210 and 220 to the mobile station 100.
That is, the HS-PDSCH is used for data transmission from the respective base stations 210 and 220 to the plurality of mobile stations 100 (for convenience of explanation, only one mobile station is shown in FIG. 9).
The base stations 210 and 220 determine a schedule on the basis of which data transmission is performed to each of the plurality of mobile stations 100, and perform data transfer at a different timing to the each mobile station 100.
In the HSDPA, each of the base stations 210 and 220 sets a DPCH (Dedicated Physical Channel), which is an individual channel, between itself and each of the plurality of mobile stations 100.
In the DPCH, there are included the downlink channels 21 and 22 through which the control information is transmitted from the base stations 210 and 220 to the mobile station 100, and the uplink channels 11 and 12 through which the control signal is transmitted from the mobile station 100 to the base stations 210 and 220.
As described above, in the HSDPA, data transmission is performed from the one base station 210 by using the high-speed physical downlink shared channel 31, while the control signal is transmitted and received to and from the plurality of base stations 210 and 220 by using the DPCH (the downlink channels 21 and 22 and the uplink channels 11 and 22).
Next, there will be described the soft handover and the hard handover, which are specified by the 3GPP as handover systems.
The soft handover is a handover system by which the mobile station 100 sets up channels simultaneously with the plurality of base stations 210 and 220. The soft handover system is applied to set up the DPCH in FIG. 8.
Each of the base stations 210 and 220 transmit a common pilot signal at predetermined power. The mobile station 100 sets up the DPCH with the base station (for example, base station 210), the reception power of the common pilot signal from which is largest. However, when the difference in the reception power is small, the mobile station 100 sets up the DPCH with the other base station (for example, base station 220), the reception power of the common pilot signal from which is relatively small.
That is, the mobile station 100 is capable of simultaneously setting up the DPCH with the plurality of base stations 210 and 220. Thereby, the mobile station 100 is capable of starting communication with the other cell (base station 220) while continuing communication with the cell (base station 210 performing the data transfer) with which the mobile station 100 is currently communicating.
On the other hand, the hard handover system is applied to the high-speed physical downlink shared channel 31 relating to the data transfer.
The hard handover is a handover for switching the base station connecting with the mobile station, according to the movement of the mobile station. The hard handover results in the current radio connection being broken between the base station and the mobile station, before a new radio connection established.
Next, there will be described the transmission power control (power control for high-speed cosed loop transmission in HSDPA) in the mobile station 100, which transmission power control is performed on the basis of control signals from the respective base stations 210 and 220 in the state where the above described soft handover is applied and the base stations 210 and 220 are simultaneously connected.
FIGS. 10A and 10B are the conceptual diagrams showing a state of the high-speed cosed loop transmission power control in the HSDPA.
FIG. 10A is a figure showing a state where the DPCH is established by the soft handover between the mobile station 100 and the plurality of base stations 210 and 220, and where the high-speed physical downlink shared channel is established by the hard handover between the base station 210 and the mobile station 100.
FIG. 10B is a figure showing control signals supplied from the base stations 210 and 220 to the mobile station 100 through the downlink channels of DPCH, and showing a state where the transmission power is adjusted in the mobile station 100.
In the transmission power control of the uplink channel of DPCH, the base stations 210 and 220 measure the reception SIR (Signal to Interference Ratio) by using an individual pilot signal included in an uplink signal, and compare the measured value with a predetermined target SIR.
In the case where the measured value is smaller than the target SIR, the TPC (Transmit Power Control) bit instructing to increase the transmission power is notified to the mobile station 100 through the downlink channels (21, 22) of DPCH, and in the other case, the TPC bit instructing to reduce the transmission power is notified to the mobile station 100 through the downlink channels (21, 22) of DPCH.
The mobile station 100 receives the TPC bit, and increases or reduces the transmission power of the uplink channels (11, 12) of DPCH according to the received TPC bit.
The transmission power control of the uplink channel is performed on the basis of the TPC bits supplied from the plurality of base stations 210 and 220 connected by the soft handover.
The respective values of the TPC bits and the increase or reduction of the transmission power in the transmission power control performed in the mobile station 100 at this time are shown in FIG. 10B.
That is, when the TPC bit sent from the base station 210 to the mobile station 100 through the downlink channel 21 of DPCH instructs to increase the transmission power in the uplink channel 11 of DPCH (denoted by “up” in the figure), and when the TPC bit sent from the base station 220 to the mobile station 100 through the downlink channel 22 of DPCH instructs to increase the transmission power in the uplink channel 12 of DPCH, that is, when both the TPC bits from the base stations 210 and 220 instruct “up”, the mobile station 100 increases the transmission power in the uplink channels 11 and 12 of DPCH (the transmission power of the mobile station 100 is denoted by “up” on the left end side in the figure).
On the contrary, when both the TPC bits from the base stations 210 and 220 instruct to reduce the transmission power (denoted by “down” in the figure), the mobile station 100 reduces the transmission power in the uplink channels 11 and 12 of DPCH (the transmission power of the mobile station 100 is denoted by “down” on the right end side in the figure).
On the other hand, when the TPC bit from one of the base stations 210 and 220 instructs to reduce the transmission power (denoted by “down” for the TPC bit from one of the base stations 210 and 220 in the figure), the mobile station 100 reduces the transmission power of the uplink channels 11 and 12 of DPCH (down).
As can be easily understood from the above, the mobile station 100 receives the TPC bit from each of the plurality of base stations. In the case where at least one TPC bit instructs to reduce the transmission power, the mobile station 100 reduces the transmission power in the uplink channels of DPCH. In the other case, (that is, when all the TPC bits instruct to increase the transmission power), the mobile station 100 increases the transmission power of the uplink channels of DPCH.
When the above described transmission power control is performed, the uplink channel reception quality satisfies the target SIR in one base station, and at the same time, it is prevented that the uplink channel reception quality deviates from the target SIR in all the base stations and that the interference wave power of the uplink channel is increased.
On the other hand, in the transmission power control of the downlink channels (21, 22) of DPCH, the mobile station 100 measures the reception SIR by using individual pilot signals included in the downlink channels (21, 22), and compares the measured value with a predetermined target SIR.
In the case where it is determined that the measured value is smaller than the target SIR by the comparison, the mobile station 100 transmits the TPC bit instructing to increase the transmission power to the base stations 210 and 220 through the uplink channels of DPCH. In the other case, the mobile station 100 transmits the TPC bit instructing to reduce the transmission power to the base stations 210 and 220 through the uplink channels of DPCH.
The base stations 210 and 220 increase or reduce the transmission power of the downlink channels (21, 22) of DPCH according to the above described values of the TPC bit transmitted through the uplink channels of DPCH.
Here, the DPCH of uplink channel is configured by a DPCCH (Dedicated Physical Control Channel) and a DPDCH (Dedicated Physical Data Channel). The DPCCH includes an individual pilot channel (Pilot), the TPC bit instructing to increase or reduce the transmission power, and an FBI (Feed Back Information) which is feedback information for phase adjustment in the transmit diversity.
Further, the DPDCH is data including user information and control information. The DPCCH and the DPDCH are quadrature modulated and multiplexed with each other, so as to be transmitted.
As described above with reference to FIGS. 10A and 10B, for transmission of data (that is, main information) which is an original object of communication between concerned parties (users) communicating with each other, a main information downlink channel based on the hard handover is connected between the corresponding one base station 210 and the one mobile station 100 by using a high-speed physical downlink shared channel. On the other hand, for transmission of sub-information such as the control signal as described above, downlink and uplink sub-information channels (DPCH) are connected on the basis of the soft handover.
As described with reference to FIG. 10B, in the transmission power control in the mobile station 100, even in the case where the TPC bit from the base station 210, which is a main branch connected to the high-speed physical downlink shared channel, instructs to increase the transmission power (in the figure, the TPC bit from the base station 210 is denoted by “up”), when the TPC bit from the base station 220, which is a sub-branch not connected to the high-speed physical downlink shared channel, instructs to reduce the transmission power (in the figure, the TPC bit from the base station 220 is denoted by “down”), the mobile station 100 preferentially follows the TPC bit (down) from the sub-branch, so as to reduce the transmission power of the uplink channels 11 and 12 of DPCH.
In this case, as for the communication between the mobile station 100 and the data signal transmitting base station (main branch) 210 which are connected with the high-speed physical downlink shared channel, despite the fact that the base station 210 issues a request for increasing the transmission power of the uplink channel 11 of DPCH, which request includes the FBI (feedback signal relating to transmission power control), there is caused an unbalance that an adjusting operation is performed in the direction to reduce the transmission power of the uplink channel of DPCH according to the TPC bit from the base station 220 which is not the main branch but is the sub-branch.
When the unbalance as described above is caused, the transmission is performed in the state where the FBI is transmitted to the base station 210 as the main branch with transmission power lower than the required transmission power, and hence an error is liable to occur (the bit error rate is increased). Thereby, the phase control in the transmit diversity, which is based on the FBI transmitted in this way, is abnormally performed. This eventually lowers the throughput of the transmission of data as the main information which is transmitted through the high-speed physical downlink shared channel.
FIG. 11 is a conceptual diagram showing a state of the transmit diversity in the state where the feedback information (FBI) includes an error.
Also in FIG. 11, similarly to the transmit diversity shown in FIG. 6 and FIG. 7, known signals transmitted as the signals TS1 and TS2 from two antennas (not shown) reach the mobile station (not shown) as the signal RS1 and the signal RS2 through the radio space which is the propagation path. The signal RS1 and the signal RS2 are synthesized in the mobile station, so that a prescribed diversity combining gain is (originally) obtained.
As described above, in the open loop transmit diversity, the amount of phase rotation in the radio propagation path is separately culculated in the mobile station for each transmission antenna, and the feedback information requiring that the signals TS1 and TS2 be transmitted as signals with phases shifted beforehand from each antenna on the basis of the culculated result is transmitted from the mobile station to the base station.
However, when the unbalance in the transmission power control is caused as described above, the erroneous feedback information itself is transmitted to the base station. Hence, on the side of the base station which receives the erroneous feedback information, the phase difference between TS1 and TS2 is erroneously set according to the erroneous feedback information.
Thus, the signals TS1 and TS2 having such unsuitable phase difference propagate in the radio space and reach the mobile station as the signal RS1 and the signal RS2. Thereby, the phases of the signal RS1 and the signal RS2 which reach the mobile station are not coincident with each other, so as to cause the diversity combining gain to be deviated.
When the cosed loop transmit diversity is in the state as described with reference to FIG. 11, there is caused, as described above, the problem that the throughput relating to the transmission of data as the main information transmitted through the high-speed physical downlink shared channel is eventually lowered.
In order to solve such problem, it is only necessary to reduce the generation of error in the transmission of feedback information by increasing the transmission power of the uplink channel of DPCH.
However, when the transmission power of the uplink channel of DPCH is unconditionally increased, the transmission power of the uplink channels of DPCH is increased more than needed. This results in a new problem that the power consumption of the mobile station is increased and that the interference wave power of the uplink channels is increased.
There has already been proposed a technique which reduces the generation of error in the transmission of feedback information by controlling the transmission power of the uplink channel of DPCH, while coping with such problem (please refer to JP2004-80235A (please refer to paragraph 0021 to paragraph 0025, and the like, and hereinafter referred to as Patent Document 1), JP2004-7030A (please refer to paragraph 0021 to paragraph 0027, paragraph 0021 to paragraph 0028, and the like, and hereinafter referred to as Patent Document 2), and the like).
In Patent Document 1 and Patent Document 2, there is disclosed a technique in which the transmission power control of an individual uplink channel is performed only on the basis of the transmission power control information included in an individual downlink channel from a packet transmitting base station.
In the technique disclosed in Patent Document 1 and Patent Document 2, in other words, when the data of main information is received from the main branch, the transmission power control is performed only in accordance with the TPC bit transmitted through the downlink control channel from the main branch, and without the influence of the TPC bit transmitted from the base station serving as the sub-branch.
On the other hand, in these days, in order to maintain the communication quality at a high level, the base stations are installed in various places. For this reason, it is not rare that the base station is particularly installed in places, such as an indoor place, and a station yard, where the installation space cannot be sufficiently secured all the time.
In the base station installed in such extremely limited space, it is not necessarily permitted to adopt a relatively large type base station which is provided with a plurality of antennas to perform the transmit diversity, and hence the cosed loop transmit diversity is not necessarily applied.
However, in the case where it is assumed that the technique as disclosed in Patent document 1 and Patent document 2 is simply applied in such actual state, the transmission power control is indiscriminately performed only in accordance with the TPC bit transmitted through the downlink control channel from the main branch at the time when the data of main information is received from the main branch.
However, in the state where the above described cosed loop transmit diversity is not performed, the phase adjustment in the transmit diversity is not principally performed on the basis of the feedback information. Thus, this state is essentially independent of the process in which the error of the feedback information itself is suppressed.
Even in this state, when it is configured such that the transmission power control of the uplink control channel is indiscriminately performed only according to the TPC bit transmitted through the downlink control channel from the main brunch at the time when the data of main information is received from the main brunch, the control on the side of the mobile station is performed such that the transmission power of the uplink control channel is unconditionally increased, as long as the TPC bit transmitted through the downlink control channel instructs to increase the transmission power. This may result in a case where the power is uselessly consumed.
Usually, one of the greatest demands for the mobile station which is driven by a battery and has a limited power supply capacity, is to suppress the power consumption as much as possible, and to thereby secure the continuous operation time after charging as long as possible. Therefore, it is a very important technical problem to suppress the useless power consumption as described above.
However, in Patent Document 1 and Patent Document 2, such actual technical problem is not considered in particular, and hence no solution for the problem is naturally disclosed and suggested.
The present invention has been made in view of the above described circumstances. An object of the present invention is to provide a mobile station and a transmission power control method in the mobile station, which make it possible to improve a power saving characteristic associated with transmission power control in the mobile station under a condition in which the transmit diversity is applied.